Chords
A chord is notes played together, built in thirds: start on a root and take every other note. Which size of third you build with, 3 or 4 half steps, is the whole difference between bright, dark, tense, and dreamy. Pick a root and a quality; everything plays.
Build a chord
Every button plays what it selects. The ruler under the notes counts the half-step gaps that set the quality.
Root
Quality
Why thirds?
Take every other note of a scale and the notes ring without crowding. That's a triad: root, third, fifth. The lower third sets the mood (4 half steps = major/bright, 3 = minor/dark); the upper third finishes the shape. Add one more third and you get the 7th chords. Compare the four triad moods on the same root: